When rock formations in the subsurface are saturated with or exposed to oil, either in rocks capable of trapping oil, or in oil migration pathways, residual oil may remain. Reservoir rock that once contained high oil saturation will generally hereinafter be referred to as a “palaeo oil zone”. The traces of remaining oil are referred to as oil shows or residual oil.
Existing techniques that detect palaeo oil zones may also detect current oil zones. Also, detection of oil accumulation and migration in the geological past is critical for the evaluation of oil exploration wells. During and after drilling a well, there are a number of ways of detecting whether rocks are currently oil saturated, or have been oil saturated. These include detecting the concentration and composition of gas in drilling mud using gas chromatography; visual inspection to detect fluorescence in core or cuttings; repeat formation tests (RFT) to make pressure measurements, drill stem tests (DST) to obtain samples of pore fluid, and geophysical measurements (logging) to detect changes in resistivity in the rock formation.
Some of these methods are too expensive to implement on every exploration well. Also, these methods cannot always adequately detect palaeo oil zones due to drainage of most of the oil, such that the residual oil is less than the sensitivity of the methods or due to obscuring of residual oil by contamination introduced during the drilling or testing processes. Oil-based drilling mud and some mud additives also fluoresce under ultraviolet light and the residual oil viewed may in fact be contamination.
Oil inclusions in rock grains have been used to indicate the presence of palaeo oil, and an abundance of oil inclusions can be used to indicate palaeo oil saturation semi-quantitatively. Fluid inclusion technology is used to evaluate palaeo oil accumulation and migration in exploration and appraisal.
One method to evaluate palaeo oil saturation using oil inclusions is outlined in U.S. Pat. No. 5,543,616, where grains containing oil inclusions are counted. Another method is fluid inclusion stratigraphy (FIS) (as set forth in U.S. Pat. No. 4,856,351 and U.S. Pat. No. 5,241,859). FIS is an analytical technique that involves automated mass spectrometer analysis of volatile compounds released by crushing samples of sedimentary rock after an aqueous based cleaning procedure. When grains containing oil inclusions are crushed the inclusion oil will be detected as volatile compounds. However, not all rocks that have been in contact with oil contain oil inclusions because the formation of oil inclusions requires healed micro-fractures or overgrowths on grains at the time of oil charge and migration.
Oil may also be retained in a rock because of incomplete drainage, for example due to capillary or wettability effects. Previous attempts to detect oil shows using fluorescence have detected residual oil surrounding grains or have measured the oil dissolved in solvent extracts from rock samples of cuttings and core obtained during drilling (eg. U.S. Pat. Nos. 4,609,821, 4,814,614; 5,049,738).